Now, it is Christ Who guides His Church, the Popes and Bishops in communion with the Pope are not arbitrary in their teachings. The Catechism teaches:

“Yet this Magisterium is not superior to the Word of God, but is its servant. It teaches only what has been handed on to it. At the divine command and with the help of the Holy Spirit, it listens to this devotedly, guards it with dedication and expounds it faithfully. All that it proposes for belief as being divinely revealed is drawn from this single deposit of faith.” [48] – CCC # 86.

The Deposit if Faith is all what is found implicitly or explicitly in Sacred Tradition and Sacred Scripture. So we cannot hold that the Catholic Church teaches “whatever She feels like teaching” which is false. Since the Church is guided by the Holy Spirit and received the authority from Christ Himself to go out and preach (John 20:22-23) (Luke 10:16) (1 Tim 3:15), She has received the charism to correctly interpret, clarify or confirm what is already in Tradition and/or Sacred Scripture in an implicit or explicit manner.

The Catechism goes on to say:

“Mindful of Christ's words to his apostles: “He who hears you, hears me”, [49] (Luke 10:16) The faithful receive with docility the teachings and directives that their pastors give them in different forms.

The dogmas of the faith

The Church's Magisterium exercises the authority it holds from Christ to the fullest extent when it defines dogmas, that is, when it proposes truths contained in divine Revelation or also when it proposes in a definitive way truths having a necessary connection with them.” – CCC #’s 87 & 88.

There are things, however, that the Church does not have authority to teach, like for example, the ordaining of women as priests. Pope St. John Paul II wrote in his Apostolic Letter Ordenatio Sacerdotalis the following:

“Wherefore, in order that all doubt may be removed regarding a matter of great importance, a matter which pertains to the Church's divine constitution itself, in virtue of my ministry of confirming the brethren (cf. Lk 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful.”

The Catechism also teaches:

“…No one can give himself the mandate and the mission to proclaim the Gospel. the one sent by the Lord does not speak and act on his own authority, but by virtue of Christ's authority; not as a member of the community, but speaking to it in the name of Christ. No one can bestow grace on himself; it must be given and offered. This fact presupposes ministers of grace, authorized and empowered by Christ…

…the ministry in which Christ's emissaries do and give by God's grace what they cannot do and give by their own powers, is called a “sacrament” by the Church's tradition. Indeed, the ministry of the Church is conferred by a special sacrament.” # 875.

So it’s false the claim which states that Catholics believe something simply because “the Church says so” in the sense that the Church can come up with something out of Her own arbitrarily and teach it as dogma of faith.